1. Field of the Invention
The present invention relates to an angular velocity sensor for measuring the angular velocity of moving bodies such as vehicles, ships, airplanes, robots and the like. The angular velocity sensor is preferably used, for example, to control an attitude of a vehicle.
2. Related Arts
Conventionally, various types of angular velocity sensors are known. For example, Japanese Patent Application Laid-Open No. 9-105634 teaches a semiconductor type angular velocity sensor fabricated by using silicon micro technique. In detail, an oscillator is formed in a semiconductor substrate by etching a predetermined region of the semiconductor substrate. A driving element for oscillating the oscillator and a detecting element for detecting angular velocity are formed on a surface of the oscillator. The driving element and the detecting element are each formed by laminating a piezoelectric film and an electrode film on the surface of the oscillator. As a result, a small angular velocity sensor can be realized by fabricating an angular velocity sensor using semiconductor manufacturing technique.
Although not taught in the above-mentioned publication, a circuit for driving the driving element by feedback control in accordance with an oscillating state of the oscillator is normally provided to augment the operation of the angular velocity sensor, in order to achieve self-oscillation. The inventors of the present invention prepared a feedback element which indicates the oscillating state of the oscillator in the above-mentioned semiconductor type angular velocity sensor and studied a case in which the driving element is driven by self-oscillation.
FIG. 8 shows a structure of the angular velocity sensor used for the experiment. FIG. 9 is a sectional view taken along a line IX--IX in FIG. 8.
In this angular velocity sensor, a semiconductor oscillator 2 having a shape of a tuning fork is formed at a center portion of a semiconductor substrate and a frame portion 1 is formed encompassing the oscillator 2, by etching a predetermined region of the semiconductor substrate such as a silicon substrate using photolithography technique.
The oscillator 2 is supported by the frame portion 1 so that Z-axis becomes an axis of the angular velocity sensor in a three-axis rectangular coordinate system which includes X-axis, Y-axis, and Z-axis as shown in FIG. 8. The oscillator 2 is formed by a pair of arms 3, 3' extending vertically in parallel to each other and a connecting portion 4 for connecting the arms 3, 3' and for jointing the oscillator 2 to the frame portion 1. Each of the arms 3, 3' is composed of a wide arm portion 5, 5', a narrow arm portion 6, 6', and a mass portion 7, 7'.
Driving elements 8, 8' for oscillating the oscillator 2 in the X-axis direction, detecting elements 9, 9' for producing signals in accordance with an oscillating state of the oscillator 2 in the Y-axis direction, and feedback elements 10, 10' for producing signals in accordance with an oscillating state of the oscillator in the X-axis direction are formed on a front surface of the oscillator 2. Electrode pads 18a, 18a' connected to the driving elements 8, 8' via wires 18b, 18b', electrode pads 19a, 19a' connected to the detecting elements 9, 9' via wires 19b, 19b', and electrode pads 20a, 20a', connected to the feedback elements 10, 10' via wires 20b, 20b' are formed on the frame portion 1. It is to be noted that these wires and electrode pads are made of aluminum.
Further, as shown in FIG. 9, insulation films 11, 11', 12, 12' are formed on the semiconductor substrate constituting the oscillator 2. Piezoelectric materials 9a, 9a' made of ZnO, PZT or the like and electrodes 9b, 9b', are sequentially laminated on the semiconductor substrate between the insulation films 11 and 12, and between the insulation films 11' and 12' by film formation using sputtering process or vapor deposition process, thereby forming the detecting elements 9, 9'. The wires 20b, 20b' connected to the feedback elements 10, 10' are formed on the insulation films 11, 11', respectively. Members forming the detecting elements 9, 9' including the wires 20b, 20b' formed on the insulation films 11, 11' are covered by protective films 13, 13', respectively. It is to be noted that each of the driving elements 8, 8' and the feedback elements 10, 10' is also formed by sequentially laminating the piezoelectric material and the electrode on the semiconductor substrate constituting the oscillator 2, in the similar manner with the detecting elements 9, 9'. The semiconductor type angular velocity sensor thus fabricated is brought into an operating state while the semiconductor substrate thereof is grounded.
The driving elements 8, 8' are located at positions at which a center line of each of the driving electrode 8, 8' running in the X-axis direction is downwardly offset from a center line of the connecting portion 4 running in the X-axis direction. For this arrangement, when alternating current voltage is applied on the electrodes of the driving elements 8, 8' so that each of the driving electrodes 8, 8' repeats expansion and contraction, the pair of arms 3, 3' can be symmetrically oscillated in the X-axis direction.
Further, as is apparent from the drawing, the feedback elements 10, 10' are located at positions offset in a right and left direction of the drawing (i.e., in the X-axis direction) from center lines of the wide arm portions 5, 5' running in the Z-axis direction. For this arrangement, when the oscillator 2 oscillates, alternating current signals having in-phase components are produced by the feedback elements 10 and 10' in response to the oscillation of the oscillator 2.
FIG. 10 shows a diagram of an electric circuit provided to the above-mentioned angular velocity sensor.
Signals produced by the feedback elements 10, 10' are added by an addition and amplification circuit 30. Because the feedback elements 10, 10' produce alternating current signals having in-phase components with respect to the oscillation in the X-axis direction, a composite signal thereof is generated from the addition and amplification circuit 30. It is to be noted that, when the pair of arms 3, 3' oscillate in directions opposite to each other along the Y-axis direction by angular velocity acting around the Z-axis, signals having negative-phase components are respectively produced by the feedback elements 10, 10' with respect to the oscillation in the Y-axis direction. However, such negative-phase components are cancelled through addition carried out by the addition and amplification circuit 30.
The driving elements 8, 8' are driven by applying the alternating current signal generated by the addition and amplification circuit 30. In this way, the driving elements 8, 8' are driven based on the signals from the feedback elements 10, 10' which detect the oscillating state of the oscillator 2 oscillated by the driving elements 8, 8'. Therefore, the driving elements 8, 8' are driven by self-oscillation.
While the driving elements 8, 8' oscillate the oscillator 2 in the X-axis direction, when angular velocity acts around the Z-axis as shown in FIG. 8, Coriolis force is generated at the mass portions 7, 7' in the Y direction. Stress in accordance with the Coriolis force is applied to the detecting elements 9, 9', whereby the detecting elements 9, 9' generate alternating current signals in response thereto. However, the alternating current signals include, in addition to signal components based on the angular velocity, offset noise caused by leakage oscillation and signal flow-around from the driving elements during the oscillation of the oscillator 2.
The alternating current signals from the detecting elements 9, 9' are differentially amplified by the differential amplification circuit 31. After that, the differentially amplified signal is fed to a synchronous detection circuit 33 via a band-pass filter (BPF) 32.
Because the alternating current signal generated from the addition and amplification circuit 30 has a phase difference of 90.degree. from the alternating current signal passing through the BPF 32, the phase of the alternating current signal from addition and amplification circuit 30 is shifted 90.degree. by a 90.degree. phase shifter 34. The synchronous detection circuit 33 synchronously detects the alternating current signal passing through the BPF 32 using the alternating current signal the phase of which is shifted by the 90.degree. phase shifter 34. Noise components other than the signal components based on the angular velocity can be cut away through the synchronous detection. The output signal from the synchronous detection circuit 33 is outputted as a direct current angular velocity signal via a low-pass filter (LPF) 35, a gain adjustment circuit 36, and a zero point adjustment circuit 37.
FIG. 11 shows a specific circuit structure of the addition and amplification circuit 30 and the differential amplification circuit 31. The addition and amplification circuit 30 is composed of buffer amplifiers 30a, 30b and an addition amplifier 30c so that the respective alternating current signals from the feedback elements 10, 10' are added. The differential amplification circuit 31 is composed of buffer amplifiers 31a, 31b and a differential amplifier 31c so that one of the alternating current signals from the detecting elements 9, 9' is subtracted from the other thereof.
In the above-mentioned angular velocity sensor, as the sensor is more downsized, the Coriolis force generated at the mass portions 7, 7' becomes smaller. Accordingly, the magnitude of the signals generated from the detecting elements 9, 9' also becomes smaller. Further, as the size of the oscillator 2 is made smaller, the oscillating frequency thereof becomes higher.
In a case where very weak and high frequency signals are to be dealt with, it is normally considered that areas of the detecting elements 9, 9' are enlarged. However, if the area of the detecting element 9, 9' is larger than that of the feedback element 10, 10', input impedance of the addition and amplification circuit 30 becomes different from input impedance of the differential amplification circuit 31. As a result, a phase difference occurs feedback signal which has been shifted 90.degree. by the 90.degree. phase shifter 34 and a detection signal passing through the BPF 32. That is, when no angular velocity is generated, as shown by wave forms in an upper part and a middle part of FIG. 12, there is caused phase difference between the feedback signal of which the phase thereof is shifted 90.degree. and the detection signal (in this case, offset signal). Due to this phase difference, a wave form of the signal after the synchronous detection is as shown in a lower part of FIG. 12. Therefore, when such a signal is converted to a direct current signal, the offset signal is generated. This offset signal varies depending on temperature of the sensor.